Observations with the electron microscope of longitudinal sections of the sciatic nerves of infant mice during the period of early myelin formation are described.

These observations are interpreted in relation to previous studies of transverse sections, and a general picture of the formation of an internodal length of the myelin sheath in three dimensions is formulated.

In general, an internodal length of myelin sheath is attained by the spiral wrapping of the infolded Schwann cell surface; the increase in length of the internode during maturation is at least partially explained by the increased length of axon covered by the overlapping of successive layers during the wrapping of the infolded Schwann cell surface; and the nodes of Ranvier refer to the structure complex at the junctions of adjacent non-syncytial Schwann cells. The fact that the mode of formation of myelin brings each of its layers into intimate contact with the axon surface at the nodes is emphasized because of the possible functional significance of this arrangement. The manner of origin of Schmidt-Lantermann clefts remains obscure.

Certain isolated observations provide evidence for the possibility that occasional internodes of myelin may form from several small segments of myelin within a single Schwann cell.